Defense Intelligence Reference Document Invisibilty Cloaking Theory And Experiments

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refractive materials than for the meta materials of cloaking devices. So the graph
indicates the possible progress toward optical cloaking.
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Figure 14. Advances in Metamaterials. The solid symbols denote materials with negative refraction; the open
symbols denote optical materials with negative magnetic response. Orange: data from structures based on the
double split-ring resonator (SRR); green: data from U-shaped SRRs; blue: data from pairs of metallic nanorods;
red: data from the "fishnet" structure. The four insets give pictures of fabricated structures in different frequency
regions. 22
On the other hand, cloaking devices require bulk metamaterials with varying cell
structures. The first moderately bulk negatively refracting materials23, 24 were made
only recently.
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The most severe practical problem of the currently discussed cloaking devices is not the
technology for manufacturing and structuring the required metamaterials but a problem
at the core of their principal design. 25
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